Sometimes optical systems, for example those associated with telescopes, microscopes and/or binoculars, are desirably configured to reverse and/or invert an image of an object.
As is well-known and illustrated in FIG. 1, a plane mirror 10 is a simple component for inverting an image I of an object O.
As is also well-known and illustrated in FIG. 2, two plane mirrors 20, 30 having respective mirrored surfaces 21, 31 are disposed at a right angle form a so-called roof mirror 40, which is a component capable of both inverting and reversing an image I of an object O.
It has proven challenging to build hollow roof mirrors accurately and robustly at reasonable cost and weight. Arranging and securing two delicate plane mirrors 20, 30 into a roof configuration can demand exacting tolerances during manufacture and assembly. Tiny gaps or misalignments tend to deform or destroy the optical path, as can any excess adhesive commonly used to retain plane mirrors 20, 30 in a roof configuration.
Plane mirrors 20, 30 may be attached to one another by an optical contact bond involving intermolecular forces. An optical contact bond may be formed by grinding and polishing abutment surfaces 22, 32 until they are highly conformal to each other, and then pressing abutment surfaces 22, 32 together. The polishing process may change the relative angles of abutment surfaces 22, 32 (e.g. relative to one another and/or relative to mirrored surfaces 21, 31), and this may affect the angle between mirrored portions 21, 31. This may make it difficult to form a desired angle between mirrored surfaces 21, 31 with the desirable degree of precision.
The problem of misaligned elements is also a concern in terms of durability. Over time, or through extended or extreme use, poorly designed elements that were originally aligned might slip out of alignment.
These manufacturing, assembly, and durability issues are important in many precision applications, such as for example, in optical components (e.g. telescopes, microscopes and/or binoculars) associated with sensitive measurement instruments. The challenges have been particularly present in manufacturing telescopes, binoculars and telescope and binocular accessories because there is a desire that telescopes and binoculars be sufficiently robust to survive use in the field. For example, there is a desire that telescopes and binoculars be able to survive vibrations during transportation to the field, survive temperature changes (and the associated thermal expansion/contraction, between storage, transportation and field environments), survive bad weather in the field, resist impact damage from hard use and the like.
One approach for attempting to satisfy these challenges has been to substitute two solid roof prisms for a hollow roof mirror. FIG. 3 shows an example of such an approach, where two solid roof prisms 50, 60 are configured into a so-called double Porro prism 70. When so configured, solid roof prisms 50, 60 take advantage of the phenomenon of total internal reflection to function as reflectors, not refractors. Light entering the solid roof prism 50 through a transmission facet 51 exits though the same transmission facet 51, after being twice internally reflected by reflection facets 52, 53. Similarly, light entering the solid roof prism 60 through a transmission facet 61 exits though the same transmission facet 61, after being twice internally reflected by reflection facets 62, 63.
Solid roof prisms and double Porro prisms have the advantage of being generally more robust than hollow roof mirrors. The reflection surfaces of the reflection facets being within the prism, the prism material is in essence being used as both reflector and frame. However, solid roof prisms also have significant disadvantages. One such disadvantage is the relative expense of manufacturing solid roof prisms, particularly in larger sizes. High-quality glass (which is itself expensive) must be carefully pressure-shaped under extreme heat and then slowly cooled. Even when great care is taken, this heating/cooling process still has a tendency to cause defects within the prism that can deform or otherwise negatively impact the optical path into, out of or through the glass. Another disadvantage associated with solid roof prisms is the typically greater weight associated with the full prism as opposed to the plane mirrors of the hollow roof mirror.
FIG. 4 illustrates another approach to satisfying these challenges which involves abutting two prisms 80, 90 together to create a hollow roof prism 100. A reflective coating is applied to the exterior surface of one facet on each prism 80, 90 (the reflection facets 81, 91) and the two reflection facets 81, 91 are disposed to form a right angle between them. Such a hollow roof prism 100 has a number of advantages. It is relatively robust and the interiors of component prisms 80, 90 need not be free of optical defects, because the optical path remains outside of component prisms 80, 90. However, the FIG. 4 hollow roof prism 100 also has disadvantages. Component prisms 80, 90 may still be more expensive and challenging to manufacture than simple plane mirrors and if component prisms 80, 90 are misaligned during assembly or use, they may not form an optically accurate roof. Roof prism 100 is also relatively heavy because of the extra material associated with component prisms 80, 90 (as compared to plane mirrors).
FIG. 5 illustrates still another approach to satisfying these challenges which involves constructing a framed hollow roof mirror 110, by disposing two plane mirrors 120, 130 at a right angle to form a roof and supporting plane mirrors 120, 130 with various frame components 140. In the case of the illustrated example, frame components 140 include: a base 141, opposing lateral supports 142 and 143, a plurality of shock-damping connectors 144, and opposing end-braces 145 (note that one of end-brace 145 has been removed from the FIG. 5 illustration so as not to obscure the other components of framed hollow roof mirror 110). While a framed hollow roof mirror benefits from the advantages that plane mirrors provide over prisms, it also suffers from the disadvantages inherent in a framing mechanism. By way of example, such disadvantages include additional weight and manufacturing and maintenance complications. Furthermore, it is generally difficult to form a fine junction between the two plane mirrors at the apex of the roof and to maintain that junction during use.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.
There remains a general need for effective apparatus and methods for roof mirrors.